The present invention relates to an exponential conversion circuit having a function of exponentially changing a gain of a variable gain amplifier on the basis of a gain control signal.
In recent years, mobile communication devices represented by a portable phone or the like are briskly developed. It is important that these mobile communication devices are small in size and light in weight because these devices are based on a presupposition that the devices can be carried by a user.
Consequently, at present, it rarely happens that these mobile communication devices comprise a combination of a plurality of individual components (functions). The mobile communication devices comprise ASIC's mixedly providing the plurality of functions. As a consequence, size reduction and weight reduction of the mobile communication devices are realized.
By the way, such mobile communication devices naturally have a sending and receiving circuit for sending and receiving electronic waves for the exchange of information by wire (electric waves). In the IF (intermediate frequency) portion of the sending and receiving circuit, a variable gain amplifier is arranged, and this variable gain amplifier has a function of adjusting an IF signal to an appropriate level.
For example, there is available a code division multiple access (CDMA) method as one of the mobile communication methods. In the CDMA method, the control of the sent electric power in a mobile station becomes indispensable, a wide scope gain control of 70 dB or more is demanded for the variable gain amplifier used in the IF portion.
Generally, in order to perform such wide scope gain control with the variable gain amplifier, it is necessary to exponentially adjust a signal level with respect to the gain control signal. Furthermore, in order to facilitate the gain control, it becomes important that the relation between the control input signal and the decibel display output signal has a linear configuration over a wide scope.
Furthermore, portable phones are based on a presupposition that the phones are carried by a user. Consequently, it is desired that the gain of the variable gain amplifier used therein has a small dependency on a temperature change resulting from a change in the environment in which the phones are used. Furthermore, a gain error must be suppressed which is caused by a disparity in a threshold value of a MOS transistor resulting from a manufacturing process of an integrated circuit.
However, for example, the constant maintenance of the characteristic for exponentially changing the gain of the variable gain amplifier with respect to the gain control signal, and the change of the decibel display output signal with respect to the control input signal become very difficult for the following reasons.
In the beginning, the variable gain amplifier will be explained.
As shown in FIG. 1, a variable gain amplifier and a gain control circuit thereof comprise a MOS transistor (CMOS circuit).
Here, the MOS transistor is generally used in the double characteristic area (strong inversion area) but can be used in a sub threshold area (weak inversion area). In this case, an exponential operation is conducted and the transmission characteristic can be approximately described in the following manner.                               I          D                =                  kx          ⁢                                           ⁢                      W            L                    ⁢          exp          ⁢                                           ⁢                                    V              GS                                      n              ⁢                                                           ⁢                              V                T                                                                        (        1        )            
Incidentally, in the expression (1), symbol Ip denotes a drain current of a MOS transistor, symbol W denotes a channel width of the MOS transistor, symbol L denotes a channel length of the MOS transistor, symbol VGS denotes a voltage between the gate and the source of the MOS transistor, and symbol VT denotes a thermal voltage. Symbol n is a constant. Furthermore, symbol Kx has a value associated with a conductance of the MOS transistor. Symbol Kx depends on the manufacturing process of the integrated circuit together with the constant n.
By the way, in FIG. 1, a variable gain amplifier 702 can vary the gain with a bias current Ibias. Furthermore, the bias current Ibias becomes equal to a drain current ID of a MOS transistor M701 with the current mirror circuits M702 and M703 inside of a gain control circuit 701.
On the other, when the MOS transistor M701 in the variable gain control circuit 701 is allowed to be operated in a weak inversion area to give a gain control signal Vc to a gate of the MOS transistor M701, the drain current ID of the MOS transistor M701 changes exponentially with the change in the gain control circuit Vc.
That is, as a consequence, the gain of the variable gain amplifier 702 changes exponentially with the change of the gain control signal Vc.
However, the following problem is generated in order to directly use the characteristic of the expression (1) in the circuit of FIG. 1.
That is, when logarithm on both sides of the equation (1) is taken, the following equation is provided.                               log          ⁢                                           ⁢                      I            D                          =                              log            ⁢                                                   ⁢            kx                    +                      log            ⁢                                                   ⁢                          W              L                                +                                    (                              1                                  n                  ⁢                                                                           ⁢                                      V                    T                                                              )                        ⁢                          V              GS                                                          (        2        )            
Here, as described above, in expression (2), symbol Kx is affected by the influence of the manufacturing process of the integrated circuit, the transmission characteristic (expression (2)) of the MOS transistor, namely, the exponential conversion characteristic changes with the manufacturing process, specifically, a disparity in the thickness and the processing generated at the time of the manufacturing process.
Furthermore, a third item on the right side of the expression (2) determines the exponential conversion characteristic (characteristic of the exponential correlation). However, with respect to a heat voltage VT, in order to maintain the temperature dependency, the exponential conversion characteristic also changes depending on the temperature change in the case where a temperature change is generated in the MOS transistors M 701, M 702 and M 703 in the gain control circuit. As a consequence, the variable scope (gain characteristic) of the gain of the variable gain amplifier 702 changes.
Incidentally, in the gain control circuit 701 of FIG. 1, even when a bipolar transistor is used instead of the MOS transistor 701, the exponential conversion characteristic (characteristic of the exponential correlation) comes to have a temperature dependency for the same reasons as described above.
Consequently, in the case where the exponential conversion characteristic of an active device is directly used in the variable gain amplifier 702, an error is generated in the exponential conversion characteristic resulting from a change in the environment (temperature change) and a manufacturing process of the integrated circuit so that a desired exponential conversion characteristic cannot be obtained.
Furthermore, with respect to the variable gain amplifier 702 used in a wireless receiver, it is important to linearly change a decibel display output signal with respect to the control input signal. Furthermore, it is required that the temperature dependency of the exponential conversion circuit is small, and the exponential conversion characteristic is not affected by the characteristic change of the active device resulting from the manufacturing process of the integrated circuit.
In this manner, in the case where exponential operation (characteristic) of the active device (MOS transistor) is directly used in the control of the variable gain amplifier 702, the variable gain amplifier 702 cannot maintain at a constant level a variable scope (gain characteristic) of the gain of the variable gain amplifier 702 with respect to the variable scope of the gain control signal Vc because of the change of the temperature environment of the active device, the characteristic change of the active device due to the manufacturing process of the integrated circuit, or the like.
From this, it is demanded that the gain is exponentially changed with respect to the gain control signal, that is, the relation between the change in the gain control signal and change in the gain which is a decibel display is linear from the viewpoint of facilitation of the control in the case where the gain is controlled in the wireless communication device. Furthermore, in such a case, it is demanded that the change ratio of the gain characteristic with respect to the temperature is definite, and the gain characteristic does not change with the characteristic change of the active device resulting from the manufacturing process of the integrated circuit with the result that the realization of the gain control circuit for realizing these demands is desired.
As described above, in the conventional variable gain amplifier and the gain control circuit for controlling the gain, there is a problem that the variable scope (gain characteristic) of the gain in the variable gain amplifier with respect to the variable scope of the gain control signal cannot be maintained on a definite level because the gain characteristic of the variable gain amplifier changes resulting from the characteristic change of the active device and the temperature change.